Elemental mercury and mercury compounds are trace contaminants in all types of fossil fuels (hydrocarbons); coal, petroleum (oil), and natural gas. Upon the gasification of coal, or the refinement of oil or natural gas, the resulting products can also contain mercury.
The gas and liquid hydrocarbon streams are many, and often are known by more than one name. In addition, the compositions of these gas or liquid hydrocarbon streams comprising mercury vary. Gas and oil wells are the commercial source of the hydrocarbon streams. Typical well gas comprises methane, ethane, propane, n-butane, iso-butane, iso-pentane, n-pentane and higher molecular weight hydrocarbons, as well as carbon dioxide, hydrogen sulfide, and nitrogen. The gas, and the gas associated with crude oil, may be treated at an extraction plant. The gas can be treated to produce “natural gas” or “sales gas”, meaning gas which is at least 75 percent methane; Liquefied Natural Gas (“LNG”), meaning natural gas which is in liquid form at −230° F.; ethane (usually 95 percent ethane); E/P, a mixture of ethane and propane; LPG, a mixture of propane and butane; Natural Gas Liquids (“NGL”), typically ethane and higher, sold as individual liquid products; and natural gasoline/condensates, a mixture of pentanes and higher hydrocarbons.
Crude oil typically contains a mixture of hydrocarbons ranging from methane to complex multi-ring aromatic compounds, including normal paraffins, which are straight chain alkanes; iso-paraffins, which are branched chain alkanes; olefins, which are hydrocarbons which contain at least one double bond with no aromatic or cycloparaffinic rings; cycloparaffins (naphthenes) such as cyclopentane, cyclohexane; mononuclear aromatics, which are hydrocarbons which have at least one benzene ring; polynuclear aromatics, which are hydrocarbons which have two or more aromatic rings; resins such as lower molecular weight asphaltene, having molecular weights of 500-1500; asphaltenes, complex high molecular weight compounds, having molecular weights of 500-100,000; and heteroatom compounds, which are compounds containing not just carbon and hydrogen but sulfur, oxygen, nitrogen, nickel, or others.
The quality of the crude oil and the particulars of the refinery unit operations will determine the particular product distribution when the crude oil is refined. Refineries perform three major steps to transform crude oil into components: separation, such as by atmospheric distillation or vacuum distillation; conversion, such as by catalytic cracking, reforming, or visbreaking; and treatment, such as hydrotreating, desulfurization, and contaminant removal. The major sources of hydrocarbon feedstocks from a refinery are atmospheric distillation and fluidized catalytic cracker (“FCC”) offgas.
In a simple refinery, the atmospheric distillation of crude oil produces light ends, which can be treated in the gas plant. In addition, naphtha and gasoline are produced, and can then be separated. The naphtha can be hydrotreated and reformed to produce gasoline. Isomerization can increase the octane rating of naphtha and gasoline. Light and heavy distillates are also products of atmospheric distillation, as are gas oil and residue. The light distillates can be hydrotreated to produce kerosene and jet fuels. The heavy distillate can be hydrotreated to produce diesel and heating oil. The gas oil and residue can be used as heavy fuel oil. In a more complex refinery, one of the products of the atmospheric distillation is atmospheric gas oil (in addition to the light and heavy distillates). The bottoms of the atmospheric column are sent to vacuum distillation where two products are produced. One product is vacuum gas oil, which can be combined with the atmospheric gas oil and further processed via catalytic cracking. The second product is the residue, which can be sent to a coker to produce gasoline and heavy fuel oil. The catalytic cracking processes produce products such as C4 and lighter olefinic gases, gasoline, light cycle oil, decanted (or slurry) oil and coke. Isobutanes and olefins can be combined via alkylation to form a gasoline blending agent.
Hydrocracking can be used to convert heavy petroleum products into gasoline and/or middle distillate fuel blending stocks. Heavy fractions such as heavy gasoline can be fed to a catalytic reformer for conversion to higher value products. The lighter fraction products can be used in the production of gasoline, and as LPG. The middle distillate is usually blended for fuel oil.
Thermal cracking operations, such as coking, delayed coking, fluid coking and visebreaking can be used to increase the value of residual products from atmospheric or vacuum distillation.
Finally, coal, a solid fossil fuel containing mercury, can be gasified, for example, as part of the conversion of coal to other chemicals (methanol, olefins, etc.).
The mercury in the hydrocarbon liquid and gas streams is highly corrosive to aluminum parts, can poison catalysts, and can present industrial hygiene and environmental risks from contaminated equipment, emissions, and waste.
A range of technologies exists to deal with the removal of mercury from hydrocarbon streams. An overview of the adsorbents was made, which defined the materials generically as:                sorbents consists of granular or pelletized material consisting of a substrate support zeolite, activated carbon, metal oxide or alumina and a reactive component Ag, KI, CuS, metal sulfide, etc. that is bonded to the support. ( . . . ) Sorbents function by reacting mercury or a mercury compound to a chemical form HgS, HgI or amalgam that is insoluble in hydrocarbon liquid and chemically inert to the components of process stream.Wilhelm, S. M. “Design mercury removal systems for liquid hydrocarbons” Hydrocarbon Processing, International Edition, 1999, 78(4), 61-66, 68-71.        
One category of mercury removal methods treats the feedstock. The advantage to these methods is that the mercury is removed upstream of the primary processing steps, which protects the remainder of the facility from the contamination. One example is the process of the Institut Francais du Petrole (Rueil-Malmaisson, FR) (“IFP Process”) which passes a liquid hydrocarbon feedstock over a nickel on alumina bed along with hydrogen. Organometallic and ionic mercury is converted to metallic mercury. The treated feed then passes over an adsorbent material made of a metal sulfide on an adsorbent where the mercury bonds to the sulfide and is retained on the support. See, e.g., U.S. Pat. No. 4,911,825.
U.S. Pat. Nos. 4,950,408 and 5,338,444 describe removing mercury from an organic medium using ion exchange resins, with thiol groups bound to a polymeric substrate. Depending on the functional group, the resin is active for mercury or mercury and arsenic. U.S. Pat. No. 5,082,569 teaches the use of similar materials in combination with a molecular sieve impregnated with silver to trap elemental mercury. U.S. Pat. No. 5,336,835 describes a process where the mercury contaminated hydrocarbon liquid is passed over a carbon bed impregnated with a metal halide. The mercury is converted from organomercury compounds to inorganic halides and/or the reduction of non-elemental mercury compounds to elemental mercury.
Another category of methods for mercury removal treat the product streams in the recovery section. The advantage of these methods is that many of the processing steps convert the various mercury components into metallic or elemental mercury. The mercury distributes itself over different product streams (as illustrated in Table 1), and the optimal technology which works best for each specific stream and plant layout can be chosen.
For methods treating particular product streams, one group of adsorbents is based on the reactivity of elemental mercury with sulfur. The mercury forms compounds such as HgS, which is a stable solid compound that can be separated from the liquid or gaseous feed. Typically the sulfur is supplied on a solid porous adsorbent, using activated carbon or aluminum oxide as carrier or as metal sulfide. See U.S. Pat. Nos. 4,500,327 and 4,708,853 (sulfur impregnated activated carbon beds and carbon molecular sieves). Calgon Carbon Corporation, Pittsburgh, Pa. describes using sulfur in its HGR® activated carbon support. Mersorb® mercury adsorbent, from Selective Adsorption Associates, Inc., Langhorne, Pa., is described as a mercury adsorbent on activated carbon.
Metal sulfides are described as being used on alumina to treat liquid and gas streams. See U.S. Pat. No. 4,094,777. Puraspec 1156, Puraspec 1157, and Puraspec 5156 (all from Johnson Matthey Catalysts, Houston, Tex.) are said to treat gas streams with pre-sulfided mixed oxides to remove mercury (Puraspec 1156), to treat gas streams to remove both hydrogen sulfide and mercury from by reaction with spherical mixed oxides (Puraspec 1157), and to remove mercury from liquid hydrocarbon steams using a spherical mixed metal sulfide absorbent (Puraspec 5156). Mechanical strength is an issue with these materials, as they are generally mostly mixed oxides with some binder, not supported on a carrier.
Another group of adsorbents is based on the affinity of mercury to form an amalgam. U.S. Pat. No. 4,874,525 describes reacting mercury with another metal, such as silver, which has been bonded to a support such as a molecular sieve. This approach can treat either liquid or gas streams. HgSIV, offered by UOP (DesPlaines, Ill., USA) is described as using silver on the molecular sieve to remove mercury and water.
U.S. Pat. No. 4,909,926 describes removing mercury from condensate using a high surface area support and a reactive adsorbent on the support, where the reactive adsorbent is reactive to mercury. The reactive adsorbent may be metallic silver and the support high surface area alumina.
Sud-Chemie (Munich, Germany) markets a product T-2552, comprised of silver on gamma-alumina for use in removing mercury from gas feedstocks. The material is a standard adsorbent which is applied in removal of contaminants both in the ethylene plant as well as in the ethylene product.
An example of one location where mercury removal is important is in ethylene plants. In an ethylene plant, a steam cracker (a furnace) breaks (“cracks”) the saturated hydrocarbons down into smaller, often unsaturated hydrocarbons. To accomplish this, the feed is diluted with steam in coiled tubes and then briefly heated in a furnace. Typically, the coil outlet temperature is between about 800° C. and about 820° C., but the temperatures may vary. Depending on the design of the furnace, the residence time can be in milliseconds, but in general, the residence time is less than about 1 second. The composition of the cracked gas depends on the composition of the feed, the hydrocarbon to steam ratio, the cracking temperature, and furnace residence time. Light hydrocarbon feeds (such as ethane, LPGs or light naphthas) give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon feeds give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil. A higher cracking temperature favors the production of ethylene and benzene, whereas a lower cracking temperature produces relatively higher amounts of propylene, C4-hydrocarbons and liquid products. Pyrolysis: Theory and Industrial Practice, Lyle F. Albright, et al., Ed., 1983, p. 76.
Since the product composition is so varied, the layout of the operations downstream of the furnace area will differ as well, but there is a generic design that matches most ethylene plants. This design is shown in FIG. 1, which identifies five different unit operations. In the feed treatment operation (1) contaminants can be removed from the feed before they enter the plant. As examples of feed treatment operations, ethane crackers may have a CO2 removal unit, and liquid crackers may have water/oil separators. In the furnace operation (2), the feed is typically heated to about 820-850° C. at low pressure to cause the hydrocarbon molecules to crack into fragments and to form ethylene, propylene and other molecules. Following the furnace operation is the quench operation (3), where the hot cracked gas is cooled and partially condensed. After quenching, the cooled and partially condensed cracked gas is sent to the compression operation (4), where it is pressurized. There may be more than one compression operation, which may or may not be immediately sequential. A contaminant removal operation (5) is performed after or between stages of compression. In the contaminant removal operation, CO2 and H2S are removed, usually through a wash with a caustic solvent. Water is also removed, usually with activated aluminas or molecular sieves. The product is then sent to the separation/recovery operation (6), where it is further treated through a series of distillations to produce purified monomers like ethylene and propylene. The separation/recovery operations typically include one or more hydrogenation reactors (8) to reduce the concentration of acetylenic components. The separation/recovery operation also comprises a “cold box” (7), where the gas temperatures are reduced to very low levels to allow separation of the light molecules.
The cracked gas from the furnace operation also includes undesirable organic impurities such as carbonyls (such as aldehydes and ketones), and dienes such as cyclopentadiene. In addition, the mercury components in the feed are converted in the furnace operation into mercury, including elemental mercury, which will remain as a volatilized metal in the cracked gas. If the mercury is left in the cracked gas it will distribute over different streams in the recovery operation, as illustrated in Table 1.
TABLE 1Olefins Unit Mercury DistributionDistributionStream(Relative %)Tail Gas2Ethylene Product8Ethane Recycle2Propylene Product1Methylacetylene/19PropadieneCatalystPropane Recycle1Butanes/Butylenes53Pyrolysis Gasoline1UnaccountedBalanceMercury contamination in ethylene plants: an overview. Reid, et al., USA. AIChE Spring National Meeting, Conference Proceedings, Atlanta, Ga., United States, Apr. 10-14, 2005 (2005).